The real-time protection should still have been working, when the registration expires you just can't get updates virus definitions or program updates. Aside from that it should still work as normal, so there is something else going on.
- Try a repair of avast:
XP - Add Remove programs, select 'avast! Anti-Virus,' click the Change/Remove button and scroll down to Repair, click next and follow.
You may need to reboot after the repair.
If that works, it has been some time since your original post and the current program version is 7.0.1426, so you could try a manual program update.
If it didn't work, then a clean install of avast free 7.0.1426 would be the next step:
- Download the latest version of avast, 7.0.1426
http://files.avast.com/iavs5x/avast_free_antivirus_setup.exe and save it to your HDD, somewhere you can find it again (if you didn't save your last download). Use that when you reinstall.
- Download the avast! Uninstall Utility, aswClear.exe
find it here and save it to your HDD (it has uninstall tools for 5.x, 6.0.x and 7.0.x).
- 1. Now uninstall avast! (using add remove programs, if you can't do that start from the next step), reboot.
- 2. run the avast! Uninstall Utility from safe mode, first for 6.x if previously installed and then for 7.0, once complete reboot into normal mode.
- 3. install the latest version, reboot.
